Optical filter, method for manufacturing same, and optical module

ABSTRACT

This optical filter  10  has an L* of at least 20 as measured by the SCE method, wherein the linear transmittance is at least 60% with respect to light the wavelength of which falls at least partially within the wavelength range of 760 nm-2,000 nm, and the temperature, at which the optical filter contracts by being heated, is at least 85° C.

TECHNICAL FIELD

The present invention relates to an optical filter, a method for producing the same, and an optical module; for example, an optical filter preferably usable as an infrared filter having a high infrared regular transmittance and a high visible light diffuse reflectance, a method for producing the same, and an optical module including a device and such an optical filter provided at a front surface of an infrared receiver of the device, the device being, for example, a sensing device or a communication device.

BACKGROUND ART

Senser technologies and communication technologies using infrared rays have been developed and put into practice. Elements that receive infrared rays are generally sensitive also to visible light. Therefore, an infrared-transmissive filter that selectively transmits only infrared rays is used. The definition of the “infrared rays” varies in accordance with the technological art. In this specification, the term “infrared rays” refers to light that includes at least light (electromagnetic waves) having a wavelength in the range not shorter than 760 nm and not longer than 2000 nm and that is used for sensing or communication. The “visible light” refers to light having a wavelength in the range not shorter than 400 nm and shorter than 760 nm.

Conventional mainstream infrared-transmissive filters exhibit a black color to absorb visible light, and therefore, have a problem of being poor in terms of design quality.

Patent Document 1 discloses an infrared receiver/transmitter including a dielectric multi-layer film transmitting infrared rays and reflecting and transmitting visible light and having a matte-finished surface. Patent Document 2 discloses an optical item for infrared communication that exhibits a white color and has an infrared transmittance of 12% or higher. In order to allow the optical item to exhibit a white color, a surface of a transparent substrate is roughened to form a microscopic concaved and convexed pattern, and visible light is scattered by Rayleigh scattering provided by the microscopic concaved and convexed pattern.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Laid-Open Patent Publication No.     2006-165493 (Japanese Patent No. 4122010) -   Patent Document 2: Japanese Laid-Open Patent Publication No.     2013-65052 (Japanese Patent No. 5756962) -   Patent Document 3: Japanese Laid-Open Patent Publication No.     2010-058091 (Japanese Patent No. 5274164)

Non-Patent Literature

-   Non-patent literature 1: M. Iwata et al., “Bio-Inspired Bright     Structurally Colored Colloidal Amorphous Array Enhanced by     Controlling Thickness and Black Background”, Adv. Meter., 2017, 29,     1605050

SUMMARY OF INVENTION Technical Problem

The infrared receiver/transmitter described in Patent Document 1 colors an external surface thereof only with the visible light reflected by the dielectric multi-layer film. Therefore, the color varies in accordance with the angle at which the infrared receiver/transmitter is viewed. There is also a problem that the dielectric multi-layer film is costly.

According to studies made by the present inventors, the following has been found out regarding a film for infrared communication using such a dielectric multi-layer film as described in Patent Document 1. In the case where a motion of a hand is captured by an infrared camera through such a film, the contour of the hand is blurred, and thus it is difficult to use the film for motion capturing. A conceivable reason for this is that the film has a low infrared regular transmittance.

For example, Patent Document 3 and Non-patent Document 1 each disclose that a fine-particle dispersion having an amorphous structure or a colloidal amorphous array expresses a bright structural color (e.g., blue) with small angle dependence. Patent Document 3 describes that a fine-particle dispersion having an amorphous structure is especially useful for a use in which light of a specific wavelength is reflected (e.g., for a color material, an infrared-reflective film, etc.).

The present invention made to solve the above-described problems has an object of providing an optical filter that is usable as an infrared-transmissive filter having a high infrared regular transmittance, generally exhibits a white color, and is highly stable thermally.

Solution to Problem

Embodiments of the present invention provide the following solution to the problem.

[Item 1]

An optical filter having a value of L* measured by an SCE method of 20 or larger,

wherein the optical filter has a regular transmittance of 60% of higher for light having a wavelength in at least a part of a wavelength range not shorter than 760 nm and not longer 2000 nm, and

wherein the optical filter is contracted by being heated at a temperature of 85° C. or higher.

[Item 2]

An optical filter having a value of L* measured by an SCE method of 20 or larger,

wherein the optical filter has a regular transmittance of 60% of higher for light having a wavelength in at least a part of a wavelength range not shorter than 760 nm and not longer 2000 nm, and

wherein a size change of the optical filter occurs from room temperature to 300° C. as monotonous increase in a tensile mode of a TMA.

[Item 3]

The optical filter of item 1 or 2, wherein a size change from room temperature to 300° C. is 5% or lower.

[Item 4]

The optical filter of any one of items 1 through 3, wherein a line representing a size change from room temperature to 300° C. in a tensile mode of a TMA is generally straight.

[Item 5]

The optical filter of any one of items 1 through 4, wherein the regular transmittance for light having a wavelength of 950 nm is 60% or higher.

[Item 6]

The optical filter of any one of items 1 through 5, wherein the regular transmittance for light having a wavelength of 1550 nm is 60% or higher.

[Item 7]

The optical filter of any one of items 1 through 6, wherein a color exhibited in the case where standard light from a D65 light source is used has x and y coordinates in ranges of 0.25≤x≤0.40 and 0.25≤y≤0.40 on a CIE 1931 chromaticity diagram.

[Item 8]

The optical filter of any one of items 1 through 7, wherein a transmittance curve of the filter in a visible light wavelength region includes a curved portion where the regular transmittance is decreased monotonously from a longer wavelength side to a shorter wavelength side, and the curved portion is shifted to the longer wavelength side as an angle of incidence is increased.

[Item 9]

The optical filter of any one of items 1 through 8, wherein the regular transmittance for light having a wavelength of 950 nm when an angle of incidence is 60° is 80% or higher of the regular transmittance when the angle of incidence is 0°.

[Item 10]

The optical filter of any one of items 1 through 9, further comprising a printed layer formed of infrared-transmissive ink.

[Item 11]

The optical filter of any one of items 1 through 10, wherein the optical filter has a three-dimensional shape.

[Item 12]

The optical filter of any one of items 1 through 11, wherein the optical filter comprises a matrix and fine particles dispersed in the matrix.

[Item 13]

The optical filter of item 12, wherein the fine particles include mono-dispersed first fine particles having an average particle diameter in a range not shorter than 80 nm and not longer than 300 nm.

[Item 14]

The optical filter of item 13, wherein the first fine particles have an average particle diameter of 150 nm or longer.

[Item 15]

The optical filter of any one of items 12 through 14, wherein the fine particles have an average value of inter-gravitational center distances of 200 nm or longer on a cross-section vertical to a planar direction of the filter.

[Item 16]

The optical filter of any one of items 12 through 15, wherein the fine particles have a coefficient of variation, of an average value of inter-gravitational center distances on a cross-section vertical to a planar direction of the filter, of 10% or higher.

[Item 17]

The optical filter of any one of items 12 through 16, wherein the fine particles have a coefficient of variation, of an average value of inter-gravitational center distances on a cross-section vertical to a planar direction of the filter, of 45% or lower.

[Item 18]

The optical filter of any one of items 12 through 17, wherein the matrix contains a resin having a crosslinked structure.

[Item 19]

The optical filter of any one of items 12 through 18, wherein the fine particles form at least a colloidal amorphous array.

[Item 20]

The optical filter of any one of items 12 through 19, wherein the fine particles has a volume fraction not lower than 6% and not higher than 60%.

[Item 21]

The optical filter of any one of items 12 through 20, wherein where the matrix has a refractive index of n_(M) and the fine particles have a refractive index of n_(P), for light having a wavelength of 546 nm for the light having a wavelength of 546 nm, |n_(M)−n_(P)| is not smaller than 0.03 and not larger than 0.6.

[Item 22]

The optical filter of any one of items 12 through 21, wherein the matrix is formed of a resin, and the fine particles are formed of an inorganic material.

[Item 23]

A method for producing the optical filter of item 22, the method comprising the steps of:

preparing a curable resin composition containing the fine particles dispersed and mixed in a curable resin;

providing a surface of a substrate with the curable resin composition, and

curing the curable resin contained in the curable resin composition provided on the surface.

[Item 24]

The method of item 23, wherein the step of providing is performed by a coating method.

[Item 25]

The method of item 23, wherein the step of providing is performed by a dip coating method.

[Item 26]

An optical module, comprising:

a device including an infrared receiver; and

the optical filter of any one of claims 1 through 22 located on a front surface of the infrared receiver of the device.

[Item 27]

The optical module of item 26, wherein the device is a sensing device, a communication device, a solar cell, a heater or a power supply device.

Advantageous Effects of Invention

Embodiments of the present invention provide an optical filter that is usable as an infrared-transmissive filter having a high infrared regular transmittance, generally exhibits a white color, and is highly stable thermally, a method for producing the same, and an optical module.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of an optical filter 10 according to an embodiment of the present invention.

FIG. 2 shows a cross-sectional TEM image of an optical filter 10A in example 1.

FIG. 3 shows a cross-sectional TEM image of an optical filter 20A in comparative example 1.

FIG. 4 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter 10A in example 1.

FIG. 5 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter 20A in comparative example 1.

FIG. 6 is an example of camera image acquired by use of a motion capture device.

FIG. 7 is an example of camera image acquired by use of the motion capture device through the optical filter 10A in example 1.

FIG. 8 is an example of camera image acquired by use of the motion capture device through the optical filter 20A in comparative example 1.

FIG. 9 shows an optical image of the optical filter 10A in example 1.

FIG. 10 shows an optical image of the optical filter 20A in comparative example 1.

FIG. 11 is a schematic view illustrating optical characteristics of the optical filter 10 according to an embodiment of the present invention.

FIG. 12 is a schematic view showing a method for measuring a diffuse transmittance of an optical filter.

FIG. 13 is a schematic view showing a method for measuring a regular transmittance of an optical filter.

FIG. 14 shows a regular transmittance spectrum of the optical filter 10A in example 1.

FIG. 15 shows an absorbance spectrum found as a difference between a diffuse transmittance spectrum and a diffuse reflectance spectrum of the optical filter 10A in example 1.

FIG. 16 shows a regular transmittance spectrum of the optical filter 20A in comparative example 1.

FIG. 17 shows an cross-sectional TEM image of an optical filter in example 2.

FIG. 18 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter in example 2.

FIG. 19 shows a regular transmittance spectrum of the optical filter in example 2.

FIG. 20 shows regular transmittance spectra of optical filters in example 3 and example 4.

FIG. 21 shows a regular transmittance spectrum of an optical filter in example 5.

FIG. 22 shows a regular transmittance spectrum of an optical filter in example 6.

FIG. 23 shows a regular transmittance spectrum of an optical filter in example 7.

FIG. 24 shows a regular transmittance spectrum of an optical filter in example 8.

FIG. 25 shows a regular transmittance spectrum of an optical filter in example 9.

FIG. 26 shows a regular transmittance spectrum of an optical filter in example 10.

FIG. 27 shows a cross-sectional TEM image of the optical filter in example 10.

FIG. 28 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter in example 10.

FIG. 29 shows a regular transmittance spectrum of an optical filter in example 11.

FIG. 30 shows a regular transmittance spectrum of an optical filter in example 12.

FIG. 31 shows a regular transmittance spectrum of an optical filter in example 13.

FIG. 32 shows a cross-sectional TEM image of an optical filter in comparative example 2.

FIG. 33 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter in comparative example 2.

FIG. 34 shows regular transmittance spectra of the optical filter in comparative example 2 (angle of incidence: 0° and 60°).

FIG. 35 shows a cross-sectional TEM image of an optical filter in comparative example 3.

FIG. 36 shows a histogram of inter-gravitational center distances of particles found from the cross-sectional TEM image of the optical filter in comparative example 3.

FIG. 37 shows a regular transmittance spectrum of the optical filter in comparative example 3.

FIG. 38 shows the angle-of-incidence dependence of each of regular transmittance spectra of the optical filter 10A in example 1 and an optical filter in comparative example A.

FIG. 39 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter 10A in example 1.

FIG. 40 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter in comparative example A.

FIG. 41 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter in example 2.

FIG. 42 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter in example 6.

FIG. 43 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter in comparative example 3.

FIG. 44A shows an optical image (visible light) representing a semi-spherical optical filter in an example.

FIG. 44B shows an infrared image of the semi-spherical optical filter in the example shown in FIG. 44A.

FIG. 45 is a graph showing size changes from room temperature to 300° C. of samples of the optical filters in example 6 and comparative example A measured in a tensile mode of a TMA.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an optical filter according to an embodiment of the present invention will be described with reference to the drawings. An optical filter according to an embodiment of the present invention is not limited to any of those described in the following examples.

An optical filter according to an embodiment of the present invention includes a matrix and fine particles dispersed in the matrix. The fine particles form at least a colloidal amorphous array, and have a regular transmittance of 60% or higher for light having a wavelength in at least a part of a wavelength range not shorter than 950 nm and not longer than 2000 nm. For example, an optical filter having a regular transmittance of 60% or higher for light having a wavelength not shorter than 950 nm and not longer than 1550 nm is obtained. The wavelength range of the light for which the optical filter has a regular transmittance of 60% or higher (wavelength range of near infrared light) is preferably, for example, not shorter than 810 nm and not longer than 1700 nm, and more preferably, not shorter than 840 nm and not longer than 1650 nm. Such an optical filter is preferably usable for, for example, an InGaAs sensor, an InGaAs/GaAsSb sensor, a CMOS sensor, an NMOS sensor, and a CCD sensor. It is preferred that the matrix and the fine particles are both transparent to the visible light (hereinafter, referred to simply as “transparent”). An optical filter according to an embodiment of the present invention may exhibit a white color.

An optical filter according to an embodiment of the present invention includes a colloidal amorphous array. The “colloidal amorphous array” is an array of colloidal particles (particle diameter: 1 nm to 1 μm) that neither have a long-distance order nor cause Bragg reflection. This sharply contrasts to the colloidal particles being distributed so as to have a long-distance order, in which case a so-called colloidal crystal (a type of photonic crystal) is formed and Bragg reflection is caused. Namely, the fine particles (colloidal particles) included in such an optical filter according to an embodiment of the present invention do not form diffraction grating.

The fine particles included in an optical filter according to an embodiment of the present invention include mono-dispersed fine particles having an average particle diameter that is at least 1/10 of the wavelength of the infrared rays. Namely, for the infrared rays having a wavelength in the range not shorter than 760 nm and not longer than 2000 nm, the average particle diameter of the fine particles is preferably at least 80 nm, more preferably at least 150 nm, and still more preferably at least 200 nm. The upper limit of the average particle diameter of the fine particles is, for example, 300 nm. The fine particles may include two or more types of mono-dispersed fine particles having different average diameters. It is preferred that each of the fine particles is generally spherical. In this specification, the term “fine particles” (plural) is also used in the sense of an array of the fine particles. The term “mono-dispersed fine particles” refers to those having a coefficient of variation (a percentage value of standard deviation/average particle diameter) of 20% or lower, preferably 10% or lower, and more preferably 1 to 5%. An optical filter according to an embodiment of the present invention uses particles having a particle diameter (diameter of the particle, volume-equivalent diameter) that is at least 1/10 of the wavelength of the infrared rays, and thus has an improved infrared regular transmittance. The principle of the optical filter according to an embodiment of the present invention is different from that of the optical item of Patent Document 2, which utilizes Rayleigh scattering.

Herein, the average particle diameter was found based on a three-dimensional SEM image. Specifically, Helios G4 UX produced by FEI Company was used as a focused ion beam scanning electron microscope (hereinafter, referred to as the “FIB-SEM”) to acquire continuous cross-sectional SEM images, the positions the continuous images were corrected, and then a three-dimensional image was re-structured. This will be described in more detail. The acquisition of the cross-sectional reflected electron images by the SEM and FIB (acceleration voltage: 30 kV) processing were repeated 100 times at an interval of 50 nm to re-structure the three-dimensional image. The obtained three-dimensional image was binarized by use of the Segmention function of analysis software (AVISO of Thermo Fisher Scientific Inc.), and an image of the fine particles was extracted. Next, the Separate object operation was performed in order to identify each of the fine particles, and the volume of each fine particle was calculated. Assuming that each particle was spherical, the volume-equivalent diameter was calculated. A value obtained by averaging the particle diameters of the fine particles was set as the average particle diameter.

In order to allow an optical filter according to an embodiment of the present invention to have a regular transmittance of 60% or higher for light having a wavelength in at least a part of the wavelength range not shorter than 760 nm and not longer than 2000 nm, eighter one of the following is adjusted: the refractive indices of the fine particles and the matrix, the average particle diameter of the fine particles, the volume fraction, the distribution (degree of aperiodicity), and the thickness.

An optical filter according to an embodiment of the present invention may exhibit a white color. Herein, the “white color” refers to a color having x and y coordinates in ranges of 0.25≤x≤0.40 and 0.25≤y≤0.40 on the CIE 1931 chromaticity diagram, with the standard light being from a D65 light source. Needless to say, a color closer to x=0.333 and y=0.333 has a higher degree of whiteness. The x and y coordinates are preferably 0.28≤x≤0.37 and 0.28≤y≤0.37, and more preferably 0.30≤x≤0.35 and 0.30≤y≤0.35. The value of L* measured by the SCE method on the CIE 1976 color space is preferably 20 or larger, more preferably 40 or larger, still more preferably 50 or larger, and especially preferably 60 or larger. A color having a value of L* of 20 or larger may generally be considered to be white. The upper limit of the value of L* is, for example, 100. The method for measuring the regular transmittance will be described below regarding the experiment examples (including examples and comparative examples).

FIG. 1 is a schematic cross-sectional view of an optical filter 10 according to an embodiment of the present invention. The optical filter 10 according to an embodiment of the present invention includes a matrix 12 transparent to visible light and transparent fine particles 14 dispersed in the matrix 12. The fine particles 14 form at least a colloidal amorphous array. The optical filter 10 may include other particles that do not disturb the colloidal amorphous array formed by the fine particles 14.

As schematically shown in FIG. 1 , the optical filter 10 has a substantially flat surface. Herein, the expression “substantially flat surface” refers to a surface that does not have a concaved and convexed structure of such a size as to scatter (diffract) or diffuse-reflect visible light or infrared rays. The optical filter 10 does not contain a cholesteric liquid crystal material (encompassing a polymeric liquid crystal material, a low-molecular weight liquid crystal material, a mixture thereof, and such a liquid crystal material mixed with a crosslinker to be, for example, crosslinked and thus solidified; widely encompassing a liquid crystal material having a cholesteric phase). The optical filter 10 is, for example, film-like, but is not limited to this.

The transparent fine particles 14 are, for example, silica fine particles. Usable as the silica fine particles are silica fine particles synthesized by, for example, a Stober method. As the fine particles, inorganic fine particles other than silica fine particles may be used. Resinous fine particles may be used. The resinous fine particles are preferably fine particles formed of at least one of, for example, polystyrene and poly(methyl methacrylate), and more preferably fine particles formed of crosslinked polystyrene, crosslinked poly(methyl methacrylate) or crosslinked styrene-methyl methacrylate copolymer. As such fine particles, for example, polystyrene fine particles or poly(methyl methacrylate) fine particles synthesized by emulsion polymerization may be used when appropriate. Alternatively, air-containing hollow silica fine particles or hollow resinous fine particles may be used. Fine particles formed of an inorganic material are advantageous in being highly resistant against heat and light. The fine particles have a volume fraction that is preferably not lower than 6% and not higher than 60%, more preferably not lower than 20% and not higher than 50%, and still more preferably not lower than 20% and not higher than 40% with respect to the entirety of the optical filter (including the matrix and the fine particles). The transparent fine particles 14 may have optical isotropy.

The matrix 12 may be formed of, for example, acrylic resin (e.g., poly(methyl methacrylate), poly(methyl acrylate)), polycarbonate, polyester, poly(diethyleneglycolbisallylcarbonate), polyurethane, epoxy resin, or polyimide, but is not limited to being formed of any of these materials. It is preferred that the matrix 12 is formed of a curable (thermosetting or thermoplastic) resin. From the point of view of mass-productivity, it is preferred that the matrix 12 is formed of a photocurable resin. As the photocurable resin, any of various (meth)acrylates is usable. It is preferred that such a (meth)acrylate contains two-functional or at least three-functional (meth)acrylate. It is preferred that the matrix 12 has optical isotropy. Use of a curable resin containing a polyfunctional monomer allows the matrix 12 to have a crosslinked structure. Therefore, the heat resistance and the light resistance are improved.

The optical filter 10 including the matrix 12 formed of a resin material may be like a flexible film. The optical filter 10 has a thickness that is, for example, not less than 10 μm and not greater than 10 mm. As long as the thickness of the optical filter 10 is, for example, not less than 10 μm and not greater than 1 mm, or further, not less than 10 μm and not greater than 500 μm, the flexibility thereof is conspicuously expressed.

In the case where silica fine particles having a hydrophilic surface are used as the fine particles, it is preferred that, for example, a hydrophilic monomer is photocured to form such silica fine particles. Examples of the hydrophilic monomer include polyethyleneglycol(meth)acrylate, polyethyleneglycoldi(meth)acrylate, polyethyleneglycoltri(meth)acrylate, polypropyleneglycol(meth)acrylate, polypropyleneglycoldi(meth)acrylate, polypropyleneglycoltri(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, acrylamide, methylenebisacrylamide, and ethoxylated bisphenol A di(meth)acrylate. The hydrophilic monomer is not limited to any of these materials. One of these monomers may be used independently, or two or more of these monomers may be used in mixture. Needless to say, such two or more monomers may include a monofunctional monomer and a polyfunctional monomer, or may include two or more polyfunctional monomers.

These monomers may be cured when appropriate by use of a photoinitiator. Examples of the photoinitiator include carbonyl compounds such as benzoinether, benzophenone, anthraquinone, thioxane, ketal, acetophenone, and the like; sulfur compounds such as disulfide, dithiocarbamate, and the like; organic peroxides such as benzoyl peroxide, and the like; azo compounds; transfer metal complexes; polysilane compounds; dye sensitizers; etc. Such a photoinitiator is contained at a content that is preferably not lower than 0.05 parts by mass and not higher than 3 parts by mass, and more preferably not lower than 0.05 parts by mass and not higher than 1 part by mass, with respect to 100 parts by mass of the mixture of the fine particles and the monomer.

Where the refractive index of the matrix to the visible light is n_(M) and the refractive index of the fine particles to the visible light is n_(P), |n_(M)−n_(P)| (hereinafter, may be referred to simply as the “refractive index difference”) is preferably not smaller than 0.01 and not larger than 0.6, and more preferably not smaller than 0.03 and not larger than 0.11. If the refractive index difference is smaller than 0.03, the scattering intensity is too weak to easily provide desired optical characteristics. If the refractive index difference is larger than 0.11, the infrared regular transmittance may be decreased. In the case where, for example, zirconia fine particles (refractive index: 2.13) and an acrylic resin are used to realize a refractive index difference of 0.6, the thickness may be decreased to adjust the infrared regular transmittance. As can be seen, the infrared regular transmittance is adjustable by, for example, controlling the thickness of the optical filter and the refractive index difference. For a certain use, the optical filter and a filter absorbing infrared rays may be used in a stacking manner. The refractive index to the visible light may be represented by, for example, the refractive index to light of 546 nm. Herein, the “refractive index” refers to a refractive index to light of 546 nm unless otherwise specified.

An optical filter according to an embodiment of the present invention may be produced by a production method including, for example, a step of preparing a curable resin composition containing fine particles dispersed and mixed in a curable resin, a step of providing a surface of a substrate with the curable resin composition, and a step of curing the curable resin contained in the curable resin composition provided on the surface. The substrate may be, for example, a glass substrate or a resin film formed of, for example, PET (polyethyleneterephthalate), TAC (triacetylcellulose) or PI (polyimide). The substrate is not limited to being formed of any of these materials. The fine particles may be dispersed and mixed in the curable resin by a known dispersion and mixing device such as a homo mixer, a homogenizer (e.g., supersonic homogenizer, high-pressure homogenizer), or the like. The step of providing the surface with the curable resin composition may be performed by any of various known methods such as, for example, a coating method (e.g., dip coating method, spray coating method, die coating method), a printing method and the like.

Hereinafter, specific experiment examples (examples and comparative examples) will be shown to describe features of a structure and optical characteristics of an optical filter according to an embodiment of the present invention. Table 1 shows structures and optical characteristics of optical filters in examples and comparative examples. Various optical filters shown in Table 1 that are different in the type of resin combined with the silica fine particles, presence/absence of a flocculant, and the dispersion and mixing method were produced.

The optical filters in examples 1 through 13 and comparative examples 1 through 3 were each formed as a film by use of the acrylic resin and the silica fine particles shown in Table 1. Used as the silica fine particles were mono-dispersed silica fine particles synthesized by a Stober method (average particle diameter: 110 nm; CV value of the particle diameters: 4.5%), (average particle diameter: 181 nm; CV value of the particle diameters: 4.7%), (average particle diameter: 221 nm; CV value of the particle diameters: 4.9%) and (average particle diameter: 296 nm; CV value of the particle diameters: 6.1%)). Herein, Hautform Silbol 220 produced by Fuji Chemical Co., Ltd. was used as the silica fine particles. The particle diameter distribution of the silica fine particles was measured by a scanning electron microscope SU3800 (manufactured by Hitachi High-Tech Corporation).

The silica fine particles were mixed and dispersed in each of acrylic monomers A through E at a predetermined content to prepare a curable resin composition. A surface of a substrate was coated with the curable resin composition by use of an applicator such that a film having a desired thickness would be obtained. The curable resin composition was cured to obtain an optical filter. The curable resin composition was cured as follows. Darocule 1173 used as the photoinitiator was incorporated into the acrylic monomer at a content of 0.2 parts by mass with respect to 100 parts by mass of the acrylic monomer, and the curable resin composition was irradiated with a UV lamp and thus cured by photopolymerization. Different types of monomer were used to form resins (polymers) having different refractive indices.

Acrylic monomers A through E will be shown below. Monomers A and E are trifunctional acrylates, monomer B and C are bifunctional acrylates, and monomer D is a monofunctional acrylate.

A: Pentaerythritoltriacrylate

B: Ethoxylated bisphenol A diacrylate (m+n=10)

C: Ethoxylated bisphenol A diacrylate (m+n=3)

D: Methoxypolyethyleneglycol #400 methacrylate

E: Trimethylolpropane EO denatured triacrylate

Acrylic monomers B and C are represented by the following chemical formula (formula 1).

With the thickness of the obtained film being d, the film was cut by a microtome along a plane at a position of d/2 to obtain a sample piece having a thickness same as the average particle diameter of the silica fine particles. In this manner, a sample for an observation with a TEM was obtained. A TEM (HT7820 produced by Hitachi High-Tech Corporation) was used to obtain a cross-sectional TEM image including images of 200 or more particles. Based on the TEM image, the fine particles were subjected to automatic identification Delaunay diagram analysis by use of image processing software Image J to find an average value (La) of inter-gravitational center distances of adjacent fine particles and a standard deviation (Ld). In addition, from the average value of the inter-gravitational center distances (also referred to as the “average inter-gravitational center distance”) and the standard deviation, the coefficient of variation (CV value of the distances) was found. Herein, the inter-gravitational center distances were found only regarding the particles having a particle diameter of 150 nm or longer. The particles having a particle diameter shorter than 150 nm were not targeted for finding the inter-gravitational center distances. As described below, these values act as indices that show whether or not the silica fine particles form a colloidal amorphous array, and also show the distribution state of the silica fine particles in the colloidal amorphous array. The lower limit of La is preferably 100 nm, more preferably 150 nm, still more preferably 175 nm, and especially preferably 200 nm. The upper limit of La is preferably 600 nm, and more preferably 500 nm.

The colloidal amorphous array included in an optical filter according to an embodiment of the present invention is characterized by the coefficient of variation of the average inter-gravitational center distance of fine particles 14. A low coefficient of variation indicates that the long-distance order is large, and in this case, a reflective color having angle dependence caused by Bragg reflection is expressed. By contrast, in the case where the coefficient of variation is high, the influence of the Mie scattering tends to be increased and the wavelength dependence of the light scattering tends to be decreased. Therefore, for an optical filter according to an embodiment of the present invention, the coefficient of variation of the average inter-gravitational center distance of the fine particles 14 is preferably not lower than 10% and not higher than 45%, more preferably not lower than 15% and not higher than 40%, still more preferably not lower than 20% and not higher than 45%, and furthermore preferably not lower than 25% and not higher than 35%.

TABLE 1 FRACTION OF AVERAGE SILICA FINE PARTICLE PARTICLES POLYMER DIAMETER VOLUME MASS REFRACTIVE THICKNESS La [nm] % % MONOMER INDEX [μm] SUBSTRATE [nm] EX. 1 221 29 40 A 1.49 100 GLASS 302 EX. 2 296 29 40 A 1.49 100 GLASS 440 EX. 3 221 34 45 A 1.49 100 GLASS — EX. 4 221 38 50 A 1.49 100 GLASS — EX. 5 221 29 40 A 1.49 400 GLASS — EX. 6 221 28 40 B 1.52 100 GLASS 304 EX. 7 221 28 40 B 1.52 100 PET — EX. 8 221 28 40 B 1.52 500 PET — EX. 9 221 6 10 A 1.49 100 GLASS — EX. 10 221 29 40 C 1.54 100 GLASS 341 EX. 11 221 29 40 D 1.46 100 GLASS — EX. 12 110/221 29 40 A 1.49 100 GLASS — EX. 13 181 28 40 B 1.52 100 GLASS — COMPAR- 221 29 40 A 1.49 100 GLASS 341 ATIVE EX. 1 COMPAR- 221 29 40 E 1.48 100 GLASS 237 ATIVE EX. 2 COMPAR- 110 29 40 A 1.49 100 GLASS 154 ATIVE EX. 3 COORDINATES ON CIE CV CHROMATICITY INFRARED REGULAR Ld VALUE L* DIAGRAM TRANSMITTANCE [%] BRAGG [nm] (Ld/La) (BCE) x y 760 nm 950 nm 1550 nm REFLECTION EX. 1 84 27.8% 59 0.30 0.33 75 88 90 ABSENT EX. 2 120  27.2% 70 0.30 0.31 45 64 89 ABSENT EX. 3 — — 60 0.31 0.32 83 90 90 ABSENT EX. 4 — — 68 0.30 0.33 86 91 90 ABSENT EX. 5 — — 81 0.31 0.33 64 85 85 ABSENT EX. 6 95 31.2% 69 0.30 0.32 79 92 92 ABSENT EX. 7 — — 70 0.31 0.33 79 90 91 ABSENT EX. 8 — — 87 0.31 0.33 50 87 86 ABSENT EX. 9 — — 24 0.31 0.33 73 82 89 ABSENT EX. 10 99 29.0% 73 0.30 0.33 40 75 89 ABSENT EX. 11 — — 13 0.28 0.29 86 88 86 ABSENT EX. 12 — — 55 0.26 0.29 70 80 85 ABSENT EX. 13 — — 63 0.28 0.32 87 91 92 ABSENT COMPAR- 168  49.3% 72 0.30 0.32 10 21 45 ABSENT ATIVE EX. 1 COMPAR- 22  9.4% 49 0.33 0.31 90 92 92 PRESENT ATIVE EX. 2 COMPAR- 45 29.4% 20 0.22 0.19 87 89 89 ABSENT ATIVE EX. 3

FIG. 2 shows a cross-sectional TEM image of an optical filter 10A in example 1, and FIG. 3 shows a cross-sectional TEM image of an optical filter 20A in comparative example 1. In the TEM image in each of the figures, white circles are silica fine particles and black circles are sites from which the silica fine particles have been dropped. In the image processing, the black circles were also treated as the silica fine particles.

It is seen that at the cross-section of the optical filter 10A shown in FIG. 2 , the silica fine particles are dispersed almost uniformly; whereas at the cross-section of the optical filter 20A shown in FIG. 3 , the silica fine particles are partially flocculated. A reason for this is that for producing the optical filter 20A in comparative example 1, polyethyleneglycol was incorporated as a flocculant in addition to acrylic monomer A. The flocculant was incorporated at a content of 0.1% by mass with respect to 100% by mass of acrylic monomer A.

Now, FIG. 4 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter 10A in example 1. FIG. 5 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter 20A in comparative example 1. Table 1 shows the average inter-gravitational center distance La (nm), the standard deviation Ld (nm), and the coefficient of variation (CV value of the distances) found from these histograms. Hereinafter, the CV value of the distances may be referred to simply as the “CV value”.

It is seen that the distribution of the silica fine particles in the optical filter 10A in example 1 is more uniform than the distribution of the silica fine particles in the optical filter 20A in comparative example 1. Regarding the optical filter 10A in example 1, Ld is 84 nm and the CV value is 27.8%, whereas regarding the optical filter 20A in comparative example 1, Ld is as large as 168 nm and the CV value is as high as 49.3%.

Now, with reference to FIG. 6 through FIG. 8 , results of a comparison between the optical filter 10A in example 1 and the optical filter 20A in comparative example 1 regarding the performance as an infrared filter will be described. FIG. 6 shows an example of camera image acquired by use of a motion capture device with no use of a filter. FIG. 7 shows an example of camera image acquired by use of the motion capture device through the optical filter 10A in example 1. FIG. 8 shows an example of camera image acquired by use of the motion capture device through the optical filter 20A in comparative example 1.

Herein, Leap Motion Controller (registered trademark) was used as the motion capture device, and a camera image of a hand located about 20 cm away from the motion capture device was acquired. The device uses infrared rays having a wavelength of 850 nm. As clearly seen from a comparison between FIG. 6 , FIG. 7 and FIG. 8 , the image acquired in the case where the optical filter 10A in example 1 is used (FIG. 7 ) is approximately as clear as the image acquired in the case where no filter is used (FIG. 6 ), whereas the image acquired in the case where the optical filter 20A in comparative example 1 is used (FIG. 8 ) is not clear and does not allow the hand to be recognized.

FIG. 9 shows an optical image of the optical filter 10A in example 1, and FIG. 10 shows an optical image of the optical filter 20A in comparative example 1. The optical filter 10A in example 1 and the optical filter 20A in comparative example 1 are each a film having a size of about 5 cm×about 10 cm located so as to cover a front surface of the device. As seen from FIG. 9 and FIG. 10 , both of the films exhibit a white color. Therefore, the optical filter 10A in example 1 is preferably usable as an infrared-transmissive filer and also has a high level of design quality because of the white color. Needless to say, a surface of the optical filter 10A in example 1 may be provided with a color or a pattern by printing or the like. The advantages of an optical filter according to an embodiment of the present invention will be described in detail below.

The optical characteristics of an optical filter may be evaluated as follows.

As shown in FIG. 11 , when incident light I₀ is incident on the optical filter 10, a part of the incident light I₀ is transmitted through the optical filter (transmitted light I_(i)), another part of the incident light I₀ is reflected at an interface (interface-reflected light R_(i)), and the remaining part of the incident light I₀ is scattered. The scattered light includes forward scattered light S_(f) output forward with respect to the optical filter 10 and rearward scattered light Sb output rearward with respect to the optical filter 10. The rearward scattered light Sb causes the optical filter 10 to exhibit a white color. A part of the incident light I₀ is absorbed by the optical filter 10, but the resin and the silica fine particles used herein have a low absorbance for light of 400 nm to 2000 nm.

FIG. 12 is a schematic view showing a method for measuring a diffuse transmittance of an optical filter. FIG. 13 is a schematic view showing a method for measuring a regular transmittance of an optical filter. As shown in FIG. 12 , with a sample (optical filter 10) being located in an opening of an integrating sphere 32, the diffuse transmittance was found as a percentage of the total intensity of the transmitted light I_(i) and the forward scattered light S_(f) with respect to the intensity of the incident light I₀. For measuring the regular transmittance, a sample (optical filter 10) was located at a position 20 cm away from the opening of the integrating sphere 32. The regular transmittance was found as a percentage of the intensity of the transmitted light I_(i) obtained in this state with respect to the intensity of the incident light I₀. The opening has a diameter of 1.8 cm, which corresponds to a solid angle of 0.025 sr. As a spectrometer, a UV-Visible/NIR Spectrometer UH4150 (produced by Hitachi High-Tech Science Corporation) was used. Table 1 shows the values of the regular transmittance for infrared rays of 760 nm, 950 nm and 1550 nm. Regular transmittance spectra of the samples are shown in FIG. 14 and the like. The presence/absence of Bragg reflection may be determined based on whether or not the regular transmittance spectrum has a dimple (local drop in the transmittance). Table 1 also shows the presence/absence of Bragg reflection.

The degree of whiteness of the rearward scattered light S_(b) was measured by use of a spectrophotometer CM-2600-D (produced by Konica Minolta Japan, Inc.). The value of L* was found by the SCE (Specular Component Exclude) method, and also the values of x and y coordinates on the CIE 1931 chromaticity diagram were found. As the value of L* is larger and the values of the x and y coordinates are closer to 0.33, the degree of whiteness is higher. These values are also shown in Table 1.

FIG. 14 shows a regular transmittance spectrum of the optical filter 10A in example 1, and FIG. 15 shows an absorbance spectrum of the optical filter 10A in example 1, which is found as a difference between a diffuse transmittance spectrum and a diffuse reflectance spectrum. As seen from FIG. 14 , the optical filter 10A in example 1 has a high infrared transmittance. The infrared transmittance of the optical filter 10A is especially high for infrared rays having a wavelength of 900 nm or longer. Regarding the absorbance spectrum shown in FIG. 15 , the absorbance of infrared rays of 1200 nm or longer is caused by the characteristic absorbance of the resin (organic compound) and is very low.

FIG. 16 shows a regular transmittance spectrum of the optical filter 20A in comparative example 1. As clearly seen from a comparison between FIG. 16 and FIG. 15 , the transmittance of the optical filter 20A in comparative example 1 is low. As seen from a comparison between example 1 and comparative example 1 with reference to Table 1, the value of L* and the values of the x and y coordinates on the CIE chromaticity diagram, which show the degree of whiteness, are not much different between example 1 and comparative example 1, but the infrared regular transmittance is significantly different between example 1 and comparative example 1. A conceivable reason for this is that in comparative example 1, there is an influence of the flocculation of the silica fine particles as indicated by the CV value of 49.3%, which is much higher than the CV value of 27.8% in example 1. Namely, it is seen that the CV value may be an index usable to improve the infrared regular transmittance.

Now, with reference to FIG. 17 through FIG. 19 , an optical filter in example 2 will be described. FIG. 17 shows a cross-sectional TEM image of the optical filter in example 2, and FIG. 18 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter in example 2. FIG. 19 shows a regular transmittance spectrum of the optical filter in example 2. In example 1, the silica fine particles having an average particle diameter of 221 nm are used. By contrast, in example 2, silica fine particles having an average particle diameter of 296 nm are used. The CV value in example 2 is generally equal to the CV value in example 1, but the infrared regular transmittance is lower in example 2 than in example 1. Namely, it is seen that the infrared regular transmittance may be controlled by controlling the average diameter of the silica fine particles.

Now, with reference to FIG. 35 through FIG. 37 , an optical filter in comparative example 3 will be described for comparison. FIG. 35 shows a cross-sectional TEM image of the optical filter in comparative example 3, and FIG. 36 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter in comparative example 3. FIG. 37 shows a regular transmittance spectrum of the optical filter in comparative example 3. Unlike in example 1 and example 2, in comparative example 3, silica fine particles having an average particle diameter of 110 nm are used. As clearly seen from the results shown in FIG. 37 and Table 1, the CV value in comparative example 3 is generally equal to the CV values in examples 1 and 2, but the infrared regular transmittance for infrared rays of 760 nm is as high as 87% and the visible light regular transmittance is also high in comparative example 3. The degree of whiteness is lower in comparative example 3 than in examples 1 and 2. From this, it is seen that the infrared regular transmittance and the degree of whiteness may be controlled by controlling the average diameter of the silica fine particles. From a comparison between example 1, example 2 and comparative example 3, it is considered to be preferred that the optical filter includes silica fine particles having an average particle diameter of 221 nm or longer.

Now, with reference to FIG. 20 , optical filters in example 3 and example 4 will be described. FIG. 20 shows regular transmittance spectra of the optical filters in examples 3 and 4. The optical filters in examples 3 and 4 respectively have volume fractions of silica fine particles of 34% and 38%, which are higher than 29% in example 1. In each of examples 3 and 4, as compared with the results in example 1, the infrared regular transmittance of infrared rays of each of 950 nm and 1550 nm and the x and y coordinates on the CIE chromaticity diagram are not much different, and the infrared regular transmittance of infrared rays of 760 nm and the value of L* are slightly higher. As seen from a comparison between the regular transmittance spectra in example 3 and example 4, as the volume fraction of the silica fine particles is increased, the wavelength of the scattered light is shifted to the shorter wavelength side and the value of L* is slightly increased.

Now, an infrared regular transmittance of an optical filter in example 9 shown in FIG. 25 will be referred to for comparison. The optical filter in example 9 has a volume fraction of silica fine particles that is as low as 6%. In example 9, as compared with the results in example 1, the infrared regular transmittance of infrared rays of 950 nm is slightly lower and the value of L* is significantly lower. A conceivable reason for this is that the intensity of the scattered light is decreased by the decrease in the volume fraction of the silica fine particles.

As can be seen, the infrared regular transmittance and the value of L* may be controlled by controlling the volume fraction of the silica fine particles.

Now, with reference to FIG. 21 , an optical filter in example 5 will be described. FIG. 21 shows a regular transmittance spectrum of the optical filter in example 5. The optical filter in example 1 has a thickness of 100 μm, whereas the optical filter in example 5 has a thickness of 500 μm. In example 5, as compared with example 1, the infrared regular transmittance is lower but the degree of whiteness is higher. The transmittance for the visible light may be decreased by increasing the thickness of the optical filter.

Now, with reference to FIG. 22 , an optical filter in example 6 will be described. FIG. 22 shows a regular transmittance spectrum of the optical filter in example 6. In example 6, a monomer different from that in examples 1 through 5 is used and a polymer having a refractive index of 1.52 is contained to form a matrix. In examples 1 through 5, the refractive index of the matrix is 1.49 and the difference thereof from the refractive index of the silica fine particles of 1.43 is 0.06. By contrast, in example 6, the refractive index difference is as large as 0.09. In example 6, as compared with example 1, the infrared regular transmittance for infrared rays of each of 760 nm, 950 nm and 1550 nm and the value of L* are all higher.

Now, with reference to FIG. 23 , an optical filter in example 7 will be described. FIG. 23 shows a regular transmittance spectrum of the optical filter in example 7. In example 6, the optical filter is formed on a glass substrate. By contrast, in example 7, the optical filter is formed on a PET film. As seen from a comparison between FIG. 23 and FIG. 22 and between the results shown in Table 1, it is considered that there is little influence of the substrate.

Now, with reference to FIG. 24 , an optical filter in example 8 will be described. FIG. 24 shows a regular transmittance spectrum of the optical filter in example 8. The optical filter in example 7 has a thickness of 100 μm, whereas the optical filter in example 8 has a thickness of 500 μm. In example 8, as compared with example 7, the infrared regular transmittance is lower but the value of L* is higher. The transmittance for the visible light may also be decreased by increasing the thickness of the optical filter.

Now, with reference to FIG. 26 through 28 , an optical filter in example 10 will be described. FIG. 26 shows a regular transmittance spectrum of the optical filter in example 10. FIG. 27 shows a cross-sectional TEM image of the optical filter in example 10. FIG. 28 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter in example 10. In example 10, a polymer having a refractive index of 1.54 is contained to form a matrix. The refractive index difference from the silica fine particles is 0.11, which is larger than the refractive index difference in example 6. In example 10, as compared with examples 1 and 6, the infrared regular transmittance is lower and the value of L* is higher.

Now, with reference to FIG. 29 , an optical filter in example 11 will be described. FIG. 29 shows a regular transmittance spectrum of the optical filter in example 11. In example 11, a polymer having a refractive index of 1.46 is contained to form a matrix. In example 11, the refractive index difference is as small as 0.03. It is seen in example 11, as compared with example 1, the infrared regular transmittance for infrared rays of 1550 nm is lower, the transmittance for light of the visible light region is higher, and the degree of whiteness is lower.

Now, with reference to FIG. 30 , an optical filter in example 12 will be described. FIG. 30 shows a regular transmittance spectrum of the optical filter in example 12. Silica fine particles included in the optical filter in example 12 include silica fine particles having an average particle diameter of 110 nm and silica fine particles having an average particle diameter of 221 nm. The volume ratio (110 nm:221 nm) is 1:1. In example 12, as compared with example 1, the infrared regular transmittance and the degree of whiteness are both lower (see Table 1). This is considered to be an influence of the silica fine particles having an average particle diameter of 110 nm being mixed (see comparative example 3 and FIG. 37 ).

Now, with reference to FIG. 31 , an optical filter in example 13 will be described. FIG. 31 shows a regular transmittance spectrum of the optical filter in example 13. Unlike the optical filter in example 6, the optical filter in example 13 uses silica fine particles having an average particle diameter of 181 nm. As seen from a comparison between FIG. 31 and FIG. 22 , the wavelength at which the regular transmittance starts increasing is shorter in example 13 than in example 6. Namely, in the case of the optical filter in example 13, the regular transmittance for light of the visible light region is slightly higher than in example 6, and as a result, the value of L* and the degree of whiteness are slightly lower than in example 6. However, the infrared regular transmittance has a high value in example 13. It is seen that the silica fine particles having an average particle diameter of 181 nm are preferably usable. From the point of view of the degree of whiteness, it is preferred to include silica fine particles having an average particle diameter of 200 nm or longer, and it is more preferred to include silica fine particles having an average particle diameter of 221 nm or longer.

Now, with reference to FIG. 32 through 34 , an optical filter in comparative example 2 will be described. FIG. 32 shows a cross-sectional TEM image of the optical filter in comparative example 2. FIG. 33 shows a histogram of the inter-gravitational center distances of the particles found from the cross-sectional TEM image of the optical filter in comparative example 2. FIG. 34 shows regular transmittance spectra of the optical filter in comparative example 2, and shows results at angles of incidence of 0° and 60°. The angle of incidence of 0° is the direction normal to a surface of an optical filter. In comparative example 2, the optical filter includes a matrix that is formed of acrylic monomer E and has a refractive index of 1.48. The refractive index of the matrix is merely different by 0.01 from that in example 1. The angle-of-incidence dependence of the regular transmittance was measured by use of the Automatic Variable Angle System provided by the UV-Visible/NIR Spectrometer UH4150 (produced by Hitachi High-Tech Science Corporation), with the angle of the surface of the sample with respect to the incident light (angle of the optical filter 10 in FIG. 11 ) being varied.

As seen from FIGS. 32 and 33 , the array of the silica fine particles in the optical filter in comparative example 2 has a long-distance order. As a result, the CV value shown in Table 1 is as low as 9.4%. Referring to FIG. 34 , the regular transmittance spectrum has a steep dimple (local drop in the transmittance) in the visible light region. This steep dimple is caused by Bragg reflection. In the optical filter in comparative example 2, the array of the silica fine particles has a structure close to a colloidal crystal having a long-distance order or to a colloidal crystal, not a colloidal amorphous array. The steep dimple in the visible light region is shifted by the angle of incidence, and thus the optical filter in comparative example 2 is seen as exhibiting a different color in accordance with the angle at which the optical filter is viewed. Therefore, in order to suppress the Bragg reflection in the visible light region, it is considered to be preferred that the CV value is 10% or higher. From the results in comparative example 1 described above, in order to suppress the flocculation of the silica fine particles, it is considered to be preferred that the CV value is 49% or lower.

In the case of the optical filters in examples 1 through 13, the regular transmittance spectrum does not include any steep dimple, and the silica fine particles form a colloidal amorphous array. The regular transmittance for light having a wavelength in at least a part of the wavelength range that is not shorter than 760 nm and not longer than 2000 nm is 60% or higher. In the case where the standard light is from a D65 light source, the x and y coordinates on the CIE 1931 chromaticity diagram are in the ranges of 0.25≤x≤0.40 and 0.25≤y≤0.40. The color is suppressed from being seen as exhibiting a different color in accordance with the angle at which the optical filter is viewed.

As clearly seen from the above, an optical filter according to an embodiment of the present invention acquires desired optical characteristics (e.g., the infrared regular transmittance and the degree of whiteness) and is suppressed from being seen as exhibiting a different color in accordance with the angle at which the optical filter is viewed, by having the following adjusted: the refractive indices of the fine particles and the matrix, the average particle diameter of the fine particles, the volume fraction, the distribution (degree of aperiodicity), and the thickness. Optical filters having different optical characteristics may be used in a stacking manner. For a certain use, for example, a filter absorbing infrared rays and an optical filter according to an embodiment of the present invention may be used together in a stacking manner. As understood from FIG. 9 , an optical filter according to an embodiment of the present invention exhibits a white color even when being used as being stacked on, for example, a filter exhibiting a black or any other color, and therefore, improves the level of design quality thereof.

An optical filter according to an embodiment of the present invention has a feature also in the angle-of-incidence dependence of the regular transmittance spectrum.

With reference to FIG. 38 through FIG. 40 , the angle-of-incidence dependence of the regular transmittance spectrum (angles of incidence: 0°, 15°, 30°, 45° and 60°) of each of the optical filter 10A in example 1 and an optical filter in comparative example A will be described. The optical filter in comparative example A is a commercially available infrared-transmissive filter produced by Tokai Optical Co., Ltd. under the name of White IR Window (https://www.tokaioptical.com/jp/product14/). The filter in comparative example A corresponds to the optical item described in Patent Document 2, and includes a dielectric multi-layer film and a PET film and has a matte-finished surface. The filter in comparative example A exhibits a white color and has a thickness of 120 μm.

FIG. 38 shows the angle-of-incidence dependence of the regular transmittance spectrum of each of the optical filter 10A in example 1 and the optical filter in comparative example A. FIG. 39 and FIG. 40 show the lines representing the angle-of-incidence dependence in example 1 and comparative example A normalized by the maximum transmittance. FIG. 39 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter 10A in example 1. FIG. 40 shows the angle-of-incidence dependence of the regular transmittance spectrum of the optical filter in comparative example A.

As seen from FIG. 38 , the regular transmittance of the optical filter 10A in example 1 is higher than that of the optical filter in comparative example A. In the case of the optical filter 10A in example 1, the decrease in the infrared regular transmittance by the increase in the angle of incidence is smaller than in the case of the optical filter in comparative example A. For example, when the angle of incidence is 0°, the infrared regular transmittance for infrared rays of 950 nm is 88%, and when the angle of incidence is 60°, the infrared regular transmittance for infrared rays of the same wavelength is 80%, which is 90% or higher of the transmittance when the angle of incidence is 0°. By contrast, in the case of comparative example A, when the angle of incidence is 0°, the infrared regular transmittance for infrared rays of 950 nm is 30%, and when the angle of incidence is 60°, the infrared regular transmittance for infrared rays of the same wavelength is 9%, which is as low as 30% of the transmittance when the angle of incidence is 0°. As can be seen, the infrared regular transmittance of the optical filter according to an embodiment of the present invention has small angle-of-incidence dependence. For example, for the infrared rays of 950 nm, the regular transmittance when the angle of incidence is 60° is 80% or higher, 85% or higher, or even 90% or higher of the regular transmittance when the angle of incidence is 0°.

Regarding the transmittance curve of the optical filter 10A in example 1 shown in FIG. 39 , the curved portion monotonously increasing from the visible light to the infrared rays is shifted to the longer wavelength side (by about 50 nm) as the angle of incidence is increased. Such characteristic angle-of-incidence dependence is also seen in the regular transmittance spectra of the optical filters in example 2, example 6 and comparative example 3 shown in FIG. 41 , FIG. 42 and FIG. 43 . Namely, the characteristic angle-of-incidence dependence that the curved portion monotonously increasing from the visible light to the infrared rays is shifted to the longer wavelength side as the angle of incidence is increased is considered to be caused by the silica fine particles included in the optical filter forming a colloidal amorphous array. By contrast, regarding the transmittance curve of the optical filter in comparative example A shown in FIG. 40 , the curved portion monotonously increasing from the visible light to the infrared rays is shifted to the shorter wavelength side (by about 100 nm) as the angle of incidence is increased. Namely, the transmittance curves show exactly the opposite tendencies to each other.

In the case of the transmittance curve of the optical filter in comparative example A, the curved portion monotonously increasing from the visible light to the infrared rays is shifted to the shorter wavelength side as the angle of incidence is increased. Therefore, there is an undesirable possibility that the optical filter transmits obliquely incident light having a short wavelength that should be blocked (light leak). By contrast, in the case of the optical filter in which the silica fine particles form a colloidal amorphous array, the transmittance for light having a shorter wavelength is decreased as the angle of incidence is increased. Therefore, there is no possibility of light leak, unlike in the case of the optical filter in comparative example A.

In the optical filter in which the silica fine particles form a colloidal amorphous array, the transmittance for light having a shorter wavelength is decreased as the angle of incidence is increased. This is caused by the intensity of the scattered light of the visible light (especially, on the longer wavelength side) being increased. Therefore, when being viewed obliquely, an optical filter according to an embodiment of the present invention may have a white luminance increased because the intensity of the diffuse-reflected light (rearward scattered light) is increased.

An optical filter according to an embodiment of the present invention may exhibit a white color as described above. Therefore, an optical filter having a variety of colors and a high level of design quality may be provided by using infrared-transmissive ink to print, for example, a letter, a pattern or a photograph on a surface of the optical filter. Namely, an optical filter according to an embodiment of the present invention may include an optical filter layer including a matrix and fine particles, and a printed layer located on the optical filter layer and formed of infrared-transmissive ink. The printed layer may be directly formed on a surface of the optical filter layer, or the printed layer formed on a surface of a transparent film may be located on the optical filter layer. As the infrared-transmissive ink, any type of known infrared-transmissive ink may be selected in accordance with the use or the wavelength of the infrared rays to be transmitted.

An optical filter according to an embodiment of the present invention, when being viewed obliquely, has the intensity of diffuse-reflected light increased, and therefore, has a white luminance increased and has the appearance of the design (visual recognizability of the design) improved.

An optical filter according to an embodiment may be a planar film as described above as an example, but may have any of various forms without being limited to this. An optical filter according to an embodiment may have a three-dimensional shape, and may be, for example, like a film having a three-dimensional shape. Specifically, for example, an optical filter may be formed on a surface of an object having a three-dimensional shape by use of a coating method. The surface of the object may have any shape, for example, may be a part of, or the entirety of, a sphere; a curved surface having any shape; a part of, or the entirety of, a surface of a polygonal object; or the like. It is preferred that the surface of the object does not scatter light.

For example, as shown in FIG. 44A and FIG. 44B, a semi-spherical optical filter may be formed. FIG. 44A shows an optical image (visible light) representing a semi-spherical optical filter in an example. FIG. 44B shows an infrared image of the semi-spherical optical filter in the example shown in FIG. 44A. The images shown in FIG. 44A and FIG. 44B are captured by use of a full hi-vision digital movie camera DVSA10FHDIR produced by Kenko Tokina Corporation. The image shown in FIG. 44A was captured under white LED illumination in a visible light mode. The image shown in FIG. 44B was captured in a dark room with only light from an infrared LED of the above-mentioned camera.

The optical filter shown in FIG. 44A and FIG. 44B has a thickness of 300 μm and was produced as follows. A surface of a semi-spherical object formed of an acrylic resin (PMMA) having a radius of 2 cm and a thickness of 1 mm was supplied with the same material as used in example 6 by dip coating. As shown in FIG. 44A, the filter is semi-spherical and is white. As shown in FIG. 44B, the filter transmits infrared rays.

An optical filter according to an embodiment of the present invention is usable for any of various uses in addition to a sensing device (e.g., infrared camera) described above as an example or a communication device, and is preferably usable for, for example, a solar cell, a heater using infrared rays or an optical power supply device using infrared rays.

For a certain use, an optical filter is required to be heat-resistant. Evaluation results on the heat resistance will be described below. For evaluating the heat resistance, the size change from room temperature to 300° C. in a tensile mode of a TMA was measured. The measuring conditions of the TMA are as follows.

Device: Discovery TMA450 (produced by TA Instruments)

Sample: 4 mm (width)×about 16 mm (length)

Measuring mode: Tensile mode

Measuring load: 2 gf

Atmospheric gas: N₂

For obtaining a sample in example 6, a film formed of an acrylic resin and silica fine particles of the optical filter in example 6 produced as described above was peeled off from the glass substrate. The sample in example 6 exhibited a white color and had a thickness of 120 μm.

FIG. 45 is a graph showing the size changes from room temperature to 300° C. in the tensile mode of the TMA.

As seen from FIG. 45 , regarding the film in example 6, the size change from room temperature (25° C.) to about 390° C. in the tensile mode of the TMA is monotonous increase (monotonous increase in a narrow sense). The line representing the size change is generally straight from room temperature to about 350° C. The size change until the size is decreased at about 390° C. is about 3.9%.

The evaluation on whether or not the size change is monotonous increase (monotonous increase in a narrow sense) is made with an assumption that the size change is represented by a curve (function) smoothly changing.

By contrast, in comparative example A, the size is decreased at 85° C. Then, the size is rapidly increased before the temperature reaches 300° C. The size decrease corresponds to that the first derivative of the smoothly changing curve of the size change shown in FIG. 45 is negative.

A conceivable reason why the size of the sample in example 6 is stable at high temperature is that the resin forming the matrix has a crosslinked structure.

The heat resistance required of an optical filter varies in accordance with the use thereof. According to an embodiment of the present invention, an optical filter of which the size change from room temperature to 300° C. in the tensile mode of the TMA is monotonous increase is obtained. The size change from room temperature to 300° C. may be 5% or lower. The line representing the size change of the optical filter according to an embodiment of the present invention from room temperature to 300° C. in the tensile mode of the TMA is generally straight up to about 390° C., and the size change does not accompany plastic deformation.

Regarding thermomechanical characteristics described above, substantially the same results are obtained as long as the film has a thickness in the range of about 100 μm to about 500 μm.

INDUSTRIAL APPLICABILITY

An optical filter according to an embodiment of the present invention is usable as an infrared-transmissive filter usable for, for example, sensor technologies, communication technologies and the like.

REFERENCE SIGNS LIST

-   -   10, 10A, 20A optical filter     -   12 matrix     -   14 fine particles 

1-27. (canceled)
 28. An optical filter having a value of L* measured by an SCE method of 20 or larger, wherein the optical filter has a regular transmittance of 60% of higher for light having a wavelength in at least a part of a wavelength range not shorter than 760 nm and not longer 2000 nm, wherein the optical filter is contracted by being heated at a temperature of 85° C. or higher, or a size change of the optical filter occurs from room temperature to 300° C. as monotonous increase in a tensile mode of a TMA, and wherein a transmittance curve of the filter in a visible light wavelength region includes a curved portion where the regular transmittance is decreased monotonously from a longer wavelength side to a shorter wavelength side, and the curved portion is shifted to the longer wavelength side as an angle of incidence is increased.
 29. The optical filter of claim 28, wherein a size change from room temperature to 300° C. is 5% or lower.
 30. The optical filter of claim 28, wherein a line representing a size change from room temperature to 300° C. in a tensile mode of a TMA is generally straight.
 31. The optical filter of claim 28, wherein the regular transmittance for light having a wavelength of 950 nm is 60% or higher.
 32. The optical filter of claim 28, wherein the regular transmittance for light having a wavelength of 1550 nm is 60% or higher.
 33. The optical filter of claim 28, wherein a color exhibited in the case where standard light from a D65 light source is used has x and y coordinates in ranges of 0.25≤x≤0.40 and 0.25≤y≤0.40 on a CIE 1931 chromaticity diagram.
 34. The optical filter of claim 28, wherein the regular transmittance for light having a wavelength of 950 nm when an angle of incidence is 60° is 80% or higher of the regular transmittance when the angle of incidence is 0°.
 35. The optical filter of claim 28, wherein the optical filter comprises a matrix and fine particles dispersed in the matrix.
 36. The optical filter of claim 35, wherein the fine particles include mono-dispersed first fine particles having an average particle diameter in a range not shorter than 80 nm and not longer than 300 nm.
 37. The optical filter of claim 36 wherein the first fine particles have an average particle diameter of 150 nm or longer.
 38. The optical filter of claim 35, wherein the fine particles have an average value of inter-gravitational center distances of 200 nm or longer on a cross-section vertical to a planar direction of the filter.
 39. The optical filter of claim 35, wherein the fine particles have a coefficient of variation, of an average value of inter-gravitational center distances on a cross-section vertical to a planar direction of the filter, of 10% or higher.
 40. The optical filter of claim 35, wherein the fine particles have a coefficient of variation, of an average value of inter-gravitational center distances on a cross-section vertical to a planar direction of the filter, of 45% or lower.
 41. The optical filter of claim 35, wherein the fine particles form at least a colloidal amorphous array.
 42. The optical filter of claim 35, wherein the fine particles has a volume fraction not lower than 6% and not higher than 60%.
 43. The optical filter of claim 35, wherein where the matrix has a refractive index of n_(M) and the fine particles have a refractive index of n_(P), both for light having a wavelength of 546 nm, |n_(M)−n_(P)| is not smaller than 0.03 and not larger than 0.6.
 44. The optical filter of claim 35, wherein the matrix is formed of a resin, and the fine particles are formed of an inorganic material.
 45. A method for producing the optical filter of claim 44, the method comprising: preparing a curable resin composition containing the fine particles dispersed and mixed in a curable resin; providing a surface of a substrate with the curable resin composition, and curing the curable resin contained in the curable resin composition provided on the surface.
 46. An optical module, comprising: a device including an infrared receiver; and the optical filter of claim 28 located on a front surface of the infrared receiver of the device.
 47. The optical module of claim 46, wherein the device is a sensing device, a communication device, a solar cell, a heater or a power supply device. 